Phenylethanolamine N-methyltransferase (EC 2.1.1.28; PNMT) catalyzes the conversion of norephinephrine to epinephrine in the adrenal medulla and brainstem. Utilizing milligram quantities of pure bovine adrenal PNMT obtained by our purification scheme, we have studied the biophysical characteristics of the methyltransferase enzyme. Studies on the photolabelling of PNMT with 3H-[methyl]-S-adenosyl-L-methionine (AdoMet) indicate the relatively rapid formation (10 min) of a covalent bond between AdoMet and PNMT under ultraviolet conditions. This labelling can be blocked by the presence of PNMT substrate, phenylethanolamine, but was unaffected by free radical scavengers. Additional work has revealed that PNMT is a glycoprotein, with preliminary findings of mannose-and/or glucose-rich sugar chain(s).